It is known that the aqueous mixtures of two polymers such as poly(ethylene glycol) (PEG) and dextran can separate spontaneously into two aqueous phases, called aqueous two-phase systems. Phase separation in aqueous solutions of polymers is an extraordinary and underexplored phenomenon. When two aqueous solutions of polymers are mixed, the resulting system is not homogeneous; rather, two discrete phases, or layers, form. These layers are ordered according to density and arise from the mutual immiscibility of the polymers for one another. In these systems, each phase predominantly consists of water (upwards of 70-90% (w/v)), while the polymer component is present in concentrations ranging from micromolar to millimolar. A low interfacial tension and rapid mass transfer of water-soluble molecules across the boundary characterize the interface between layers.
Previous studies of partitioning between aqueous phases have been limited to biphasic systems of immiscible polymers or inorganic salts and have focused on applications in protein chemistry, cell partitioning, and manufacturing. These Aqueous Two-Phase Systems (“ATPS”) are exemplified by the poly(ethylene glycol)-dextran, dextran-Ficoll systems, and a poly(ethylene glycol) system comprising (NH4)2SO4.
Although numerous biphasic systems have been reported, there have been relatively few reports of polymer systems that exhibit multiphase, e.g., more than two phases, separation: (i) poly(ethylene glycol)-dextran-Ficoll, (ii) Triton X100-poly(ethylene glycol)-dextran, and (iii) poly(propylene glycol)-poly(ethylene glycol)-dextran-Ficoll. The four-phase system, however, is not entirely an aqueous system because the liquid poly(propylene glycol) that was employed in the assay is insoluble in water. Thus, while polymeric, this “four” phase system can be compared to the incorporation of an organic solvent or perfluorinated alkane into a three-phase system.